Some flint show a series of roughly parallel lines, either as three dimensional ridges or a surface pattern. This lines vary in how far apart they are and how straight or curved and they may change direction or end abruptly. These are banded flints. They are often mistaken for fossils.

The cause of banded flint is still not known for certain. One theory is that in some flints the silica was originally deposited rhythmically in bands that differed in their original water content, which caused areas of the flint to weather differently to create this shape. Other theories relate to the movement of water through flints after their original formation changing the shape.

Many of the flint nodules that are mistaken for fossils do actually have a different kind of fossil origin. Many elongate, tapering flint nodules, which may also be branching, have that shape because silica gel flowed into the burrows of animals such as small shrimp in the ancient seabed. This silica gel later hardened to form flint nodules in the shape of the burrows. These are a type of trace fossil, meaning fossils that preserve the behaviour of creatures (such as footprints or burrows) rather than body parts. Flint burrow infills are commonly mistaken for fossil teeth and claws!

Left: A tapering flint nodule that may be a burrow infill, which was brought in for identification as a suspected fossil tooth.

Sponges in flint

Some flint nodules can be very spherical, and these are often mistaken for fossil eggs! Many of the spherical flints have a fossil origin, having formed around sponges. Fossil sponges in flint can also be other shapes such as elongate or club-shaped. The fossil sponge can sometimes be seen in tact within the flint nodule, but often the sponge breaks down to leave a hollow with a distinctive “fluffy” texture. This is known as a “rotten” sponge in flint. If a flint nodule has a hollow centre, you can sometimes hear loose material in the centre rattle when you shake it!

Flint geodes

Sometimes after a sponge has broken down, flint nodules have later mineral growth within the hollows. This happens when the flint lies in water with dissolved mineral content. Over long periods of time the water penetrates the flint and chemical conditions cause the deposition of the dissolved mineral within the water around the hollow inside. This forms a geode, a term for a crystal lined cavity. The crystals in flint are usually the common mineral quartz.

Hi Fiona - thanks for the replies. Just a few things - I wasn't sure about the stone that you say is a banded flint but having seen an almost identical item in a museum in Oulton Broad today saying it was an agragate with fossil shell imprint I thought what I had is the same. If this is the case then it sounds as though they have mis identified their rock!

Also are you saying that picture 3 (the one I have called 'sponge') is a flint burrow infill, a sponge in flint or a flint geode. I'm a little confused?